HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumdividual slave during one cycle. Updated data of the running processcan be found in the master as well as in the slave after one cycle. Ina fully expanded system with 31 slaves, the cycle time is about 5-ms.The cyclic polling method is deterministic. This means that after5 ms, updated sensor data are available to the control unit and isthen transmitted from the control unit to the actuators.System OperationThe AS-i slave integrated circuit (IC) allows sensors, actuatorsand modules to link up as slaves to the AS-i bus. The sensor or actuatorgets its power from the line, recognizes the data the mastersends out and then sends the data back to the master.During each cycle, 4 bits of data are transferred from the masterto each slave and then back to the master. The ports can be configuredas inputs, outputs, or bidirectional. Each time a chip is installedin a slave, the I/O configuration sets the port’s configuration, whichis a 4-bit, permanently stored value.AS-i can be scaled beyond the 4 bits of data per cycle to slaveswith more than 4 bits of information. To accomplish this, the datatransmission is spread out over several cycles. AS-i can also be usedfor analog sensors and actuators.The As-i chip may be built into sensors or actuators to make anintegrated device, or it can be built into a module where sensors, actuators,and switches are connected.The AS-i master is a central device that organizes the exchangeof data in the network and connects it to a higher level control unit.Three types of masters may be used. Pure masters allow a direct linkto a PC. Couplers allow the connection to a field bus. Stand-alonemasters include some control functions that let it work without ahigher control.The AS-i master is self-configuring and is configured when theaddresses and the individual I/O configurations and ID codes of theconnected slaves are written into the master. The master automaticallyperforms all noncyclic functions that are needed for parameters,automatic addressing and network supervision.©2001 by The Fairmont Press, Inc. All rights reserved.